Wolves are hard to train and never really lose their wild streak despite being so genetically similar to dogs, but new research reveals why wolves remain wild while puppies and their adult dog relatives are so loving and trusting of humans.

Much comes down to how they enter the world.

Evolutionary biologist Kathryn Lord of the University of Massachusetts and colleagues, in a scientific first, discovered that wolf pups are still blind and deaf when they begin to walk and explore their environment at age two weeks.

"No one knew this about wolves, that when they begin exploring they're blind and deaf and rely primarily on smell at this stage, so this is very exciting," Lord was quoted as saying in a press release.

"When wolf pups first start to hear, they are frightened of the new sounds initially," she added, "and when they first start to see they are also initially afraid of new visual stimuli. As each sense engages, wolf pups experience a new round of sensory shocks that dog puppies do not."

Puppies, in contrast, begin to explore and walk after all three senses-smell, hearing and sight-are functioning. Instead of fearing stimuli, they seem to love it and even seek out new adventures, as anyone with a puppy happily nipping at their toes has found out.

"It's quite startling how different dogs and wolves are from each other at that early age, given how close they are genetically," Lord said. "A litter of dog puppies at two weeks are just basically little puddles, unable to get up or walk around. But wolf pups are exploring actively, walking strongly with good coordination and starting to be able to climb up little steps and hills."

It then sounds like there is a tradeoff between world readiness and fear and aversion. Dogs, due to their association with humans, usually get an easier, more protected start. Puppies have time to grow and develop in guarded care. Wolves, on the other hand, have to nearly hit the ground running, able to escape predators and avoid other threats.

Lord and her team came to their conclusions after studying the responses of 7 wolf pups and 43 dogs to both family and new smells, sounds and visual stimuli.

But why are wolf pups, puppies, and adults of these species so differently behaved, when their genes are very similar?

At the gene level, Lord explains, "the difference may not be in the gene itself, but in when the gene is turned on. The data help to explain why, if you want to socialize a dog with a human or a horse, all you need is 90 minutes to introduce them between the ages of four and eight weeks. After that, a dog will not be afraid of humans or whatever else you introduced. But with a wolf pup, achieving even close to the same fear reduction requires 24-hour contact starting before age three weeks, and even then you won't get the same attachment or lack of fear."